The invention generally relates to holography, and particularly relates to the readout of holograms with improved diffraction efficiency by using a resonant system in the path of the reference beam of the hologram.
Holograms are typically recorded as a result of interference between two mutually coherent light beams, the signal beam and the reference beam. The signal beam carries the information, typically in the form of amplitude modulation imprinted on the wavefront. The reference beam interferes with the signal beam creating an interference pattern that is then recorded in photosensitive material. In the simplest case, the reference is a plane wave. On reproducing the reference beam originally used to record the hologram, one is able to reproduce the signal beam as a result of diffraction from the previously recorded interference pattern.
It is known that multiple holograms may be superimposed or multiplexed in volume media. Individual holograms may be accessed selectively in a way similar to the individual detection of multiple periodicities in crystal lattices using Bragg diffraction. The Bragg selectivity property of volume holograms forms the basis of most of the current applications of volume holograms. These include volume holographic memories, in which several holograms are multiplexed so as to yield high storage capacities, opto-electronic interconnections for telecommunications and artificial neural networks, and four dimensional (spatial and spectral) imaging.
The light efficiency of a hologram is measured by a unitless quantity called the diffraction efficiency xcex7, defined as the ratio of the diffraction power divided by the incident power. If the diffraction efficiency is low, then the aforementioned applications are limited by various factors including signal to noise ratio considerations. For example, although photorefractive crystals are rewritable, they typically yield low diffraction efficiencies before non-linear effects set in to affect the recording process. The maximum achievable xcex7 depends on the holographic materials, but conventional holographic materials having high diffraction efficiency are not typically suitable for certain applications. For example, photopolymers afford high diffraction efficiencies, but are difficult to maintain and control, and may exhibit material shrinkage. Photorefractive polymers afford high diffractive efficiencies, but are inconvenient to use since they require voltages in the order of MV/cm during the recording process. This requirement limits the useful hologram thickness (and thereby the information capacity) in practical applications. These holograms also violate the Born approximation and their behavior is qualitatively different from that of weak holograms. For example, they typically exhibit increased crosstalk between Bragg-multiplexed holograms due to re-diffraction among multiple Born orders.
A principle constraint in the practical realization of most applications of volume holograms, therefore, is that the diffraction efficiency yielded by currently available holographic recording media suitable for volume holography is very low. The diffracted beams obtained from these volume holograms are relatively weak thus rendering them unsuitable for many applications.
There is a need, therefore, for a system and method of improving the diffraction efficiency of holograms, and in particular volume holograms.
The invention provides a resonator system for use in illuminating the input to a diffractive element. The system includes a source of an electromagnetic field having a wavelength of xcex, and first and second optical elements, each of which is at least partially reflecting. The first and second optical elements are separated from one another such that the optical path between the optical elements has a distance             (                        2          ⁢          m                +        1            )        ⁢          λ      4        ,
wherein m is an arbitrary integer. In certain embodiments, the diffractive element is a hologram, and the first and second optical elements are mirrors.